1. Field of the Invention
This invention relates to pneumatic cylinders and control valves for such cylinders. It is particularly applicable to cylinders for use in, for example, spot welding machines, piercing machines, powered jigs or clamps.
2. Description of the Prior Art
There already exist various types of pneumatic cylinders with either internal or external flow controls and cushioning devices.
For some types of heavy duty production operations these typical designs do not meet specific production needs. This is especially true of pneumatic cylinders used in resistance spot welding and some piercing or clamping operations.
For example, in resistance spot welding an air cylinder with an electrode and associated equipment attached to the piston rod is typically expected to stroke forward, contact the component and reach full welding force in 0.2 seconds. It must precisely hold this force for the welding and cooling period of typically 0.4 seconds and during this period rapidly follow up at constant force the collapse or indentation of the components during welding.
To achieve this fast and then stable force condition it is usual to ensure that air can be supplied to and exhausted from the power cylinder very quickly. In addition, the end of the forward stroke is determined not by the cylinder but by the contact point of the electrode which changes during its production life as the electrode is consumed, worn, re-profiled or replaced.
This combination of rapid force build up and variable end of stroke positioning make traditional cushioning and speed control devices difficult to apply, and without them the shock loads, noise, wear and vibration of tooling and equipment in use is considerable.
One such cylinder is shown in U.S. Pat. No. 5,174,191. In that cylinder, the above-described functions are achieved at the penalty of using a large number of components, leading to a complex and expensive device.